Apparatus for metering at least one type of electrical power over a predetermined range of service voltages

ABSTRACT

Methods and apparatus for supplying power for use in metering electrical energy over a wide range of voltages with a single meter are disclosed. The wide ranging meter includes a processing unit for processing divided input voltage and a current component in order to determine electrical energy metering values. The processing unit is operable in response to supply voltages. A power supply, connected to receive the undivided voltage component, generates the supply voltages over the wide dynamic range. It is especially preferred for the power supply to include a transformer having first, second and third windings, wherein the undivided voltage component is provided to the first winding and wherein the second winding defines the output of the power supply. A switching member is connected to the first winding for permitting and preventing the flow of current in response to a control signal. A control member generates the control signal in response to the output of the power supply. It is also preferred for the control signal to disable the witch member. It is further preferred for the power supply to include a voltage blocking clamp, connected to the transformer for blocking the voltage applied to the transformer. It is still further preferred for an oscillator to be used to generate an oscillating signal for switching the switching member ON and OFF so that the switching member is provided a substantially constant OFF time.

RELATED APPLICATION DATA

[0001] This application is a divisional application of U.S. patentapplication Ser. No. 09/047,479, filed Mar. 25, 1998, which is acontinuation of U.S. patent application Ser. No. 08/478,605, filed Jun.7, 1995, now U.S. Pat. No. 5,903,145, which is a continuation of U.S.patent application Ser. No. 08/384,398, filed Feb. 3, 1995, now U.S.Pat. No. 5,457,621, which is a continuation of U.S. patent applicationSer. No. 08/259,116, filed Jun. 10, 1994, now abandoned, which is acontinuation of U.S. patent application Ser. No. 07/839,967, filed Feb.21, 1992, now abandoned.

FIELD OF INVENTION

[0002] The present invention relates generally to the field of electricutility meters. More particularly, the present invention relates toelectronic utility watthour meters or meters utilized to meter real andreactive energy in both the forward and reverse directions.

BACKGROUND OF THE INVENTION

[0003] Electric utility companies and power consuming industries have inthe past employed a variety of approaches to metering electrical energy.Typically, a metering system monitors power lines through isolation andscaling components to derive polyphase input representations of voltageand current. These basic inputs are then selectively treated todetermine the particular type of electrical energy being metered.Because electrical uses can vary significantly, electric utilitycompanies have requirements for meters configured to analyze severaldifferent nominal primary voltages. The most common of these voltagesare 120, 208, 240, 277 and 480 volts RMS. Presently, available metershave a different style for each of these applications, bothelectro-mechanical and electronic. This forces the electric utilitycompanies to inventory, test and maintain many different styles ofmeters. Consequently, a need exists for reducing the number of metertypes a utility need inventory by providing a meter capable of operationover a wide dynamic range.

[0004] The problem of wide amperage dynamic range was addressed in U.S.Pat. No. 3,976,941—Milkovic. It was there recognized that solid stateelectronic meters were becoming more desirable in metering applications,however, such solid state meters had a critical drawback in theiramperage dynamic range. An effort was described to improve the amperagedynamic range of solid state meters so that such meters would beoperationally equivalent to prior electromechanical meters. The problemwith such meters, however, was their failure to address the multiplevoltage situation. Utility companies utilizing such meters would stillbe forced to inventory, test and maintain many different styles ofmeters in order to service the various voltages provided to customers.

[0005] It has been recognized in various meter proposals that the use ofa microprocessor would make metering operations more accurate. It willbe understood, however, that the use of a microprocessor requires theprovision of one or more supply voltages. Power supplies capable ofgenerating a direct current voltage from the line voltage have been usedfor this purpose. Since electric utility companies have requirements forvarious nominal primary voltages, it has been necessary to provide powersupplies having individualized components in order to generate themicroprocessor supply voltages from the nominal primary voltage.

[0006] Consequently, a need exists for a single meter which is capableof metering electrical energy associated with nominal primary voltagesin the range from 96 to 528 volts RMS. Applicants resolve the aboveproblems through the use of a switching power supply and voltagedividers. It will be recognized that switching power supplies are known.However, the use of such a power supply in an electrical energy meter isnew. Moreover, the manner of the present invention, the particular powersupply construction and its use in an electrical energy meter is novel.

[0007] It will also be noted, in order to solve the inventory problem,designing a wide voltage range meter in the past involved the use ofvoltage transformers to sense line voltage. A significant problemassociated with the use of such transformers was the change in phaseshift and the introduction of non-linearities that would occur over awide voltage range. It was not to remove such a widely changing phaseshift or to compensate for the non-linearities.

[0008] Consequently, a need still exists for a single meter which iscapable of metering electrical energy associated with nominal primaryvoltages that also minimizes phase shift in the voltage sensors over awide voltage range.

SUMMARY OF THE INVENTION

[0009] The present invention is directed to a power supply for anapparatus for metering at least one type of electrical power over apredetermined range of service voltages supplied by electrical serviceproviders, where the apparatus comprises a voltage input circuitconnected to receive a voltage component, and a processing unit. Thepower supply comprises a surge protection circuit which receives aninput voltage, a rectifier circuit which receives an alternating currentvoltage from the surge protection circuit and outputs a rectif ieddirect current voltage, a transformer which receives the rectifieddirect current voltage at a first winding so that current flows throughthe first winding, and a second winding defines an unregulated outputvoltage of the power supply, a switching device for permitting andpreventing the flow of current through the first winding in response toa control signal, and a controller for generating the control signalbased on the voltage across the third winding. The control signal outputby the controller operates to disable the switching member.

[0010] According to another feature of the present invention, the outputof the power supply is input to a linear regulator, which outputs aregulated voltage. The regulated voltage is less than the outputvoltage, and the regulated voltage is output to a precision voltagereference generator. The unregulated voltage is input to the apparatusto determine the presence of a power fail condition.

[0011] According to yet another feature, the power supply comprises anon-volatile supply, and the regulated voltage is input to thenon-volatile supply, such that the apparatus is switched to thenon-volatile supply when the regulated voltage is not present.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will be better understood, and its numerousobjects and advantages will b come apparent to thos skilled in the artby reference to the following detailed description of the invention whentak n in conjunction with the following drawings, in which:

[0013]FIG. 1 is a block diagram of an electronic meter constructed inaccordance with the present invention;

[0014]FIG. 2 is a schematic diagram of the resistive dividers shown inFIG. 1;

[0015]FIG. 3 is a schematic diagram of the linear power supply shown inFIG. 1;

[0016]FIG. 4 is a block diagram of the power supply shown in FIG. 1;

[0017]FIG. 5 is a schematic diagram of the control and switching membersshown in FIG. 4;

[0018]FIG. 6 is a schematic diagram of the startup/feedback shown inFIG. 4; and

[0019]FIG. 7 is a schematic diagram of the voltage clamp shown in FIG.4.

DETAILED DESCRIPTION

[0020] A new and novel meter for metering electrical energy is shown inFIG. 1 and generally designated 10. It is noted at the outset that thismeter is constructed so that the future implementation of higher levelmetering functions can be supported. Meter 10 is shown to include threeresistive voltage divider networks 12A, 12B, 12C: a first processor—anADC/DSP (analog-to-digital converter/digital signal processor) chip 14:a second processor—a microcontroller 16 which in the preferredembodiment is a Mitsubishi Model 50428 microcontroller: three currentsensors 18A, 18B, 18C; a 12V switching power supply 20 that is capableof receiving inputs in the range of 96-528V; a 5V linear power supply22: a non-volatile power supply 24 that switches to a battery 26 when 5Vsupply 22 is inoperative; a 2.5V precision voltage reference 28; aliquid crystal display (LCD) 30; a 32.768 kHz oscillator 32; a 6.2208MHz oscillator 34 that provides timing signals to chip 14 and whosesignal is divided by 1.5 to provide a 4.1472 MHz clock signal tomicrocontroller 16; a 2 kByte EEPROM 35; a serial communications line36; an option connector 38: and an optical communications port 40 thatmay be used to read the meter. The inter-relationship and specificdetails of each of these components is set out more fully below.

[0021] It will be appreciated that electrical energy has both voltageand current characteristics. In relation to meter 10 voltage signals areprovided to resistive dividers 12A-12C and current signals are inducedin a current transformer (CT) and shunted. The output of CT/shuntcombinations 18A-18C is used to determine electrical energy.

[0022] First processor 14 is connected to receive the voltage andcurrent signals provided by dividers 12A-12C and shunts 18A-18C. As willbe explained in greater detail below, processor 14 converts the voltageand current signals to voltage and current digital signals, determineselectrical energy from the voltage and current digital signals andgenerates an energy signal representative of the electrical energydetermination. Processor 14 will always generate a watthour delivered(Whr Del) and, watthour received (Whr Rec), depending on the type ofenergy being metered, will generate either a volt amp reactive hourdelivered (Varhr Del)/a volt amp reactive hour received (Varhr Rec)signal or volt amp hour delivered (Vahr Del)/volt amp hour received(Vahr Rec) signal. In the preferred embodiment, each transition onconductors 42-48 (each logic transition) is representative of themeasurement of a unit of energy. Second processor 16 is connected tofirst processor 14. As will be explained in greater detail below,processor 16 receives the energy signal(s) and generates an indicationsignal representative of said energy signal.

[0023] It will be noted again that meter 10 is a wide range metercapable of metering over a voltage range from 96-528V. The componentswhich enhance such a wide range meter include the divider network12A-12C, which as previously noted are connected to receive the voltagecomponent. The dividers generate a divided voltage, wherein the dividedvoltage is substantially linear voltage with minimal phase shift overthe wide dynamic range, i.e. 96-528 Volts. A processing unit (processors14 and 16) are connected to receive the divided voltage and the currentcomponent. The processing unit processes the divided voltages and thecurrent components to determine electrical energy metering values. Itwill be appreciated from the following description that processors 14and 16 require stable supply voltages to be operable. A power supply,connected to receive the voltage component and connected to processors14 and 16, generate the necessary supply voltages from the Phase Avoltage component over the wide dynamic range. Power supply 20 couldalso run off of phase B and phase C voltages or a combination of theabove. However, a combination embodiment would require additionalprotection and rectifying components.

[0024] In relation to the preferred embodiment of meter 10, currents andvoltages are sensed using conventional current transformers (CT's) andresistive voltage dividers, respectively. The appropriate multiplicationis accomplished in a new integrated circuit, i.e. processor 14.Processor 14 is essentially a programmable digital signal processor(DSP) with built in multiple analog to digital (A/D) converters. Theconverters are capable of sampling multiple input channelssimultaneously at 2400 Hz each with a resolution of 21 bits and then theintegral DSP performs various calculations on the results. For a moredetailed description of Processor 14, reference is made to U.S. Pat. No.5,555,508, which is incorporated herein by reference and which is ownedby the same assignee as the present application.

[0025] Meter 10 can be operated as either a demand meter or as atime-of-use (TOU) meter. It will be recognized that TOU meters arebecoming increasingly popular due to the greater differentiation bywhich electrical energy is billed. For example, electrical energymetered during peak hours will be billed differently than electricalenergy billed during non-peak hours. As will be explained in greaterdetail below, first processor 14 determines units of electrical energywhile processor 16, in the TOU mode, qualifies such energy un ts inrelation to the time such units were determined, i.e. the season as wellas the time of day.

[0026] All indicators and test features are brought out through the faceof meter 10, either on LCD 30 or through optical communications port 40.Power supply 20 for the electronics is a switching power supply feedinglow voltage linear supply 22. Such an approach allows a wide operatingvoltage range for meter 10.

[0027] In the preferred embodiment of the present invention, theso-called standard meter components and register electronics are for thefirst time all located on a single printed circuit board (not shown)defined as an electronics assembly. This electronics assembly housespower supplies 20, 22, 24 and 28, resistive dividers 12A-12C for allthree phases, the shunt resistor portion of 18A-18C, oscillator 34,processor 14, processor 16, reset circuitry, EEPROM 35, oscillator 32,optical port components 40, LCD 30, and an option board interface 38.When this assembly is used for demand metering, the billing data isstored in EEPROM 35. This same assembly is used for TOU meteringapplications by merely utilizing battery 26 and reprogramming theconfiguration data in EEPROM 35. The additional time-of-use billing datais stored in the internal RAM of processor 16, which RAM is backed bybattery 26.

[0028] Consider now the various components of meter 10 in greaterdetail. Primary current being metered may be sensed using conventionalcurrent transformers. The shunt resistor portion of devices 18A-18C arelocated on the electronics assembly.

[0029] The phase voltages are brought directly to the electronicassembly where resistive dividers 12A-12C scale these inputs toprocessor 14. In the preferred embodiment, the electronic components arereferenced to the vector sum of each line voltage for three wire deltasystems and to earth ground for all other services. Resistive divisionis used to divide the input voltage so that a very linear voltage withminimal phase shift over a wide dynamic range can be obtained. This incombination with a switching power supply allows the wide voltageoperating range to be implemented.

[0030] Referring briefly to FIG. 2, each resistive divider consists oftwo 1 Meg, 1/2 watt resistors 50/52, 54/56 and 58/60, respectively.Resistors 50-60 are used to drop the line voltage at an acceptable wattloss. Each resistor pair feeds a resistor 62, 64 and 66, respectively.Resistors 62-66 are metal film resistors having a minimal temperaturecoefficient. This combination is very inexpensive compared to othervoltage sensing techniques. Resistors 50-60 have an operating voltagerating of 300 Vrms each. These resistors have been individually testedwith the 6 kV IEEE 587 impulse waveforms to assure that the resistanceis stable and that the devices are not destroyed. Resistors 62-66 scalesthe input voltage to be less than 1 Volt peak to peak to processor 14.Resistors 62-66 should be in the range of from about 100 ohms to about 1K ohms to assure this maximum voltage and maintain maximum signal.

[0031] On grounded, three wire delta systems, those components of theelectronics assembly operating on logic voltage levels (including thebattery connector) can be at an elevated voltage. In such situations,the two, 1 Meg resistor combinations (50/52, 54/56, 58/60) providecurrent limiting to the logic level electronics. The worse case currentoccurs during testing of a 480 V, 3 wire delta meter with single phaseexcitation.

[0032] It will be appreciated that energy units are calculated inprocessor 14 primarily from multiplication of voltage and current. Thepreferred embodiment of processor 14, referenced above as beingdescribed in U.S. Pat. No. 5,555,508, includes three analog to digitalconverters. The necessity for three converters is primarily due to theabsense of voltage transformers, present in prior meters.

[0033] The M37428 microcontroller 16 is a 6502 (a traditional 8 bitmicroprocessor) derivative with an expanded instruction set for bit testand manipulation. This microcontroller includes substantialfunctionality including internal LCD drivers (128 quadraplexedsegments), 8 kbytes of ROM, 384 bytes of RAM, a full duplex hardwareUART, 5 timers, dual clock inputs (32.768 kHz and up to 8 MHz), and alow power operating mode.

[0034] During normal operation, processor 16 receives the 4.1472 MHzclock from processor 14 as described above. Such a clock signaltranslates to a 1.0368 MHz cycle time. Upon power fail, processor 16shifts to the 32.768 kHz crystal oscillator 32. This allows low poweroperation with a cycle time of 16.384 kHz. During a power failure,processor 16 keeps track of time by counting seconds and rippling thetime forward. Once processor 16 has rippled the time forward, a WITinstruction is executed which places the unit in a mode where only the32.768 kHz oscillator and the timers are operational. While in this modea timer is setup to “wake up” processor 16 every 32,768 cycles to counta second.

[0035] Consider now the particulars of the power supplies shown inFIG. 1. As indicated previously, the off-line switching supply 20 isdesigned to operate over a 96-528 VAC input range. It connects directlyto the Phase A voltage alternating current (AC) line and requires noline frequency transformer. A flyback converter serves as the basis ofthe circuit. A flyback converter is a type of switching power supply.

[0036] As used herein, the “AC cycle” refers to the 60 Hz or 50 Hz inputto power supply 20. The “switching cycle” refers to the 50 kHz to 140kHz frequency at which the switching transformer of power supply 20operates. It will be noted that other switching cycle frequences can beused.

[0037] Referring now to FIG. 4, power supply 20 for use in electronicmeters includes a transformer 300 having primary and secondary windings.The input voltage (Phase A Voltage) is provided to the primary windingso that current may flow therethrough. As will be appreciated from FIG.5, the secondary winding defines the output of the power supply.Referring back to FIG. 4, a switching member 302 is connected to theprimary winding of transformer 300. Switching member 302 permits andprevents the flow of current through the primary winding. Switch member302 is operable in response to a control signal, which control signal isgenerated by control circuit 304. Controller 304 generates the controlsignal in response to a limit signal generated by the start/feedbackcircuit 306 in response to the output of power supply 20. Voltage clamp308 serves to limit the voltage applied to transformer 300 and switch302. Surge protection circuit 309 is provided at the input to protectagainst surges appearing in the Phase A voltage.

[0038] Referring now to FIG. 5, transformer 300 and switch 302 are shownin greater detail. It will be appreciated that switch 302 is atransistor. At the beginning of each switching cycle, transistor 302“turns on”, i.e. becomes conductive, and magnetizes the core oftransformer 300 by applying voltage across the primary 310. At the endof each cycle, transistor 302 turns off and allows the energy stored inthe core of transformer 300 to flow to the output of the power supply,which “output” can be generally defined by secondary 312.Simultaneously, energy flows out of the bootstrap or tertiary winding314 to power the control circuitry 304.

[0039] Feedback circuit 306 and controller 304 control the output ofpower supply 20 by varying the ON time of transistor 302. Controller 304will be described in greater detail in relation to FIG. 5. Transistor302 is connected through inverter 316 to receive the output of anoscillator formed from inverters 318, 320 and 322. It will be recognizedthat such inverters form a ring oscillator. The oscillator has afree-run frequency of 50 KHz. The ON time of transistor 302 may varybetween 200 ns and 10 μs. The OFF time is always between 8 and 10 μs.During operation, the bootstrap winding 314 of transformer 300 (pins 10and 11) powers controller 304, but this power is not available until thepower supply has started. The control circuit is a current-moderegulator.

[0040] At the beginning of a switching cycle, transistor 302 is turnedON by the oscillator output. If left alone, transistor 302 would also beturned OFF by the oscillator output. Transistor 302 remains ON until thecurrent in primary 310 of transformer 300 (pins 8 and 13) ramps up tothe threshold current level I_(th) represented as a voltage V_(th). Aswill be explained below, V_(th) is generated by feedback circuit 306.When the primary current of transformer 300, represented as a voltageV_(t) and sensed by resistor 326, ramps up to the threshold levelV_(th), pin 1 of comparator 324 terminates the ON period of theoscillator by forcing the oscillator output HIGH, which output in turnis inverted by inverter 316, shutting OFF transistor 302. Transistor 302then turns OFF until the next switching cycle. Since the V_(th)indirectly controls the ON time of transistor 302, controller 304regulates the output voltage of the power supply by comparing the sensedcurrent in transformer 300 to this threshold level.

[0041] Transistor 362 and pin 7 of comparator 326 can disable theoscillator. Transistor 362, described in greater detail in FIG. 7,disables the oscillator when the line voltage exceeds 400 volts.Comparator 328 disables the oscillator when the controller 304 hasinsufficient voltage to properly drive transistor 302. The voltage incontroller 304, VC, will be described in relation to FIG. 5.

[0042] Consider now feedback circuit 306, shown in FIG. 6. Whenconnected to the Phase A Voltage, resistor 330 slowly charges capacitor332. The high value of resistor 330 and the 400 volt limit by voltageclamp 308 limit the power dissipation of resistor 330. After a fewseconds, capacitor 332 charges above 13 volts. Transistors 334 and 336then provide positive feedback to each other and snap ON. Controller 304can run for tens of milliseconds from the charge stored in capacitor332. Normally, power supply 20 will successfully start and begin topower itself in this period. If it fails to start, transistors 334 and336 turn OFF when the charge across capacitor 332 drops below 8.5 voltsand capacitor 332 again charges through resistor 330. This cycle repeatsuntil the supply starts.

[0043] With high input voltages and without resistor 338 (FIG. 5), thecurrent sourced by resistor 330 can hold the control and start-upcircuits in a disabled state that does not recycle. When Capacitor 332drops below 8.5 volts, resistor 338 places a load on the control circuitsupply. This load insures that the start-up circuit recycles properlywith high input voltages.

[0044] As indicated above, when the primary current of transformer 300sensed by resistor 326 ramps up to the threshold level V_(th), pin 1 ofcomparator 324 can terminate the ON period of the oscillator. When thevoltage on capacitor 332 is less than 13 volts, zener diode 340 providesno voltage feedback. Under these conditions, the base-emitter voltage oftransistor 336 sets the current threshold I_(th) to about 650 mA. Thismaximum current limit protects transistor 302, as well as thosetransistors in voltage clamp 306, and prevents transformer 300 fromsaturating.

[0045] As the voltage on capacitor 332, which is representative of theoutput voltage of the supply, approaches the proper level, zener diode340 begins to conduct and effectively reduces the current threshold,i.e. effectively reduces V_(th). Each switching cycle will thentransfers less power to the output, and the supply begins to regulateits output.

[0046] When the regulating circuitry requires ON times of transistor 302less than about 400 ns, the current sense circuitry does not have timeto react to the primary current of transformer 300. In that case, theregulating circuit operates as a voltage-mode pulse width modulator.Resistor 342 (FIG. 5) generates a negative step at pin 3 of comparator324 at the beginning of each switching cycle. The regulator feedbackvoltage at pin 2 of comparator 324, which contains little currentinformation at the beginning of each switching cycle, translates thestep at pin 3 into various input overdrives of comparator 324, therebydriving the output of comparator 324 to a logic HIGH level. Thepropagation time of the comparator 324 decreases with increasingoverdrive, i.e. as the negative step increases, and the circuit acts asa pulse width modulator. The negative step will increase due to thechanging level of V_(th).

[0047] Any leakage inductance between the bootstrap winding (pins 10 and11 of transformer 300) and the output winding (pins 3 and 4 oftransformer 300) causes inaccurate tracking between the voltage oncapacitor 332 and the output voltage of the supply. This leakageinductance can cause poor load regulation of the supply. The bootstrapand output windings are bifilar wound; they are tightly coupled, havelittle leakage inductance, and provide acceptable load regulation. Sincethe two windings are in direct contact, the bootstrap winding requiresTeflon insulation to meet the isolation voltage specifications. A 100%hi-pot test during manufacture insures the integrity of the insulation.

[0048] Consider now the details of voltage clamp 308, shown in FIG. 7. A528 VAC input corresponds to 750 VDC after rectification. Switchingtransistors that can directly handle these voltages are extremelyexpensive. By using the voltage clamp of the present invention,relatively inexpensive switching transistors can be utilized.

[0049] In power supply 20, the switching member 302 is shut down duringparts of the AC cycle that exceed 400 volts. The switching transistor,transistor 302, in conjunction with two other transistors 344 and 346,can hold off 750 VDC. During surge conditions, these three transistorscan withstand over 1500 volts. In the preferred embodiment, transistors302, 344 and 346 are 600-volt MOSFETs.

[0050] Because high-voltage electrolytic capacitors are expensive andlarge, this voltage clamp 308 has no bulk filter capacitor after thebridge rectifier 348. Without a bulk filter capacitor, this switchingconverter must shut down during parts of the AC cycle. It intentionallyshuts down during parts of the AC cycle that exceed 400 volts, and noinput power is available when the AC cycle crosses zero. The 2200 μFoutput capacitor 350 (FIG. 5), provides output current during theseperiods.

[0051] As discussed above, transistors 344 and 346 act as a voltageclamp and limit the voltage applied to switching member 302. At a 528VAC line voltage, the input to the clamping circuit reaches 750 volts.During lightning-strike surges, this voltage may approach 1500 volts.When the voltage at the output of bridge rectifier 348 exceeds 400volts, zener diodes 352 and 354 begin to conduct. These diodes, alongwith the 33 KΩ resistors 356, 358 and 360, create bias voltages fortransistors 344 and 346. Transistors 344 and 346 act as source followersand maintain their source voltages a few volts below their gatevoltages.

[0052] If, for example, the output of bridge rectifier 348 is at 1000volts, the gates of transistors 344 and 346 will be at approximately 400and 700 volts respectively. The source of transistor 344 applies roughly700 volts to the drain of 346; the source of 346 feeds about 400 voltsto switching member 302. Transistors 344 and 346 each drop 300 voltsunder these conditions and thereby share the drop from the 1000 voltinput to the 400 volt output, a level which the switching converter 302can withstand.

[0053] As zener diodes 352 and 354 begin to conduct and as transistors344 and 346 begin to clamp, transistor 362 turns ON and shuts down theswitching converter. Although transistors 344 and 346 limit the voltagefed to the converter to an acceptable level, they would dissipate anexcessive amount of heat if the switching converter 302 consumed powerduring the clamping period.

[0054] When switching converter 302 shuts down, transistor 302 no longerhas to withstand the flyback voltage from transformer 300. Resistor 364takes advantage of this by allowing the output voltage of the clamp toapproach 500 volts (instead of 400 volts) as the input to the clampapproaches 1500 volts. This removes some of the burden from transistors344 and 346.

[0055] Zener diodes 352 and 354 are off and the converter 302 runs whenthe output of bridge rectifier 348 is below 400 volts. During theseparts of the AC cycle, the 33 KΩ resistors 356, 358 and 360 directlybias the gates of transistors 344 and 346. The voltage drop acrosstransistors 344 and 346 is then slightly more than the thresholdvoltages of those transistors along with any voltage drop generated bythe channel resistance of those transistors.

[0056] During the off time of transistor 302, about 10 μS, the 33 KΩresistors can no longer bias the gates of transistors 344 and 346. Diode366 prevents the gate capacitance of transistors 344 and 346 and thejunction capacitance of zeners 368 and 370 from discharging whentransistor 302 is off. This keeps transistors 344 and 3460N and ready toconduct when transistor 302 turns ON at the next switching cycle. If thegates of transistors 344 and 346 had discharged between switchingcycles, they would create large voltage drops and power losses duringthe time required to recharge their gates through the 33 KΩ resistors.

[0057] In the preferred embodiment, two 33 KΩ resistors are used inseries to obtain the necessary voltage capability from 966 surface-mountpackages.

[0058] This power supply must withstand an 8 KV, 1.2×50 μS short-branchtest. Varistor 372, resistors 374, 376 and 378, and capacitor 380protect the power supply from lightning strike surges.

[0059] A 550 VAC varistor 372 serves as the basis of the protectioncircuit. It has the lowest standard voltage that can handle a 528 VACinput. The device has a maximum clamping voltage of 1500 volts at 50amps.

[0060] A varistor placed directly across an AC line is subject toextremely high surge currents and may not protect the circuiteffectively. High surge currents can degrade the varistor and ultimatelylead to catastrophic failure of the device. Input resistors 374 and 376limit the surge currents to 35 amps. This insures that the clampingvoltage remains below 1500 volts and extends the life of the varistor totens of thousands of strikes.

[0061] Resistor 378 and capacitor 380 act as an RC filter. The filterlimits the rate of voltage rise at the output of the bridge rectifier.The voltage clamping circuit, transistors 344 and 346, is able to trackthis reduced dv/dt. Current forced through diodes 382, 384 and capacitor386 (FIG. 5) is also controlled by the limited rate of voltage rise.

[0062] Resistors 374 and 376 are 1 watt carbon composition resistors.These resistors can withstand the surge energies and voltages. Resistor378 is a flame-proof resistor that acts as a fuse in the event of afailure in the remainder of the circuit.

[0063] The values of resistors 374, 376 and 378 are low enough so thatthey do not interfere with the operation of the power supply ordissipate excessive amounts of power.

[0064] Finally it is noted that resistors 388 and 390 act to generatethe power fail voltage PF.

[0065] By using the wide voltage ranging of the invention, a singlemeter can be used in both a four wire wye application as well as in afour wire delta application. It will be recognized that a four wiredelta application includes 96V sources as well as a 208V source. In thepast such an application required a unique meter in order to accomodatethe 208V source. Now all sources can be metered using the same meterused in a four wire wye application.

[0066] While the invention has been described and illustrated withreference to specific embodiments, thos skilled in the art willrecognize that modification and variations may be made without departingfrom the principles of the invention as described herein above and setforth in the following claims.

What is claimed is:
 1. A power supply for an apparatus for metering atleast one type of electrical power over a predetermined range of servicevoltages supplied by electrical service providers, said apparatuscomprising a voltage input circuit connected to receive a voltagecomponent and a processing unit, said power supply comprising: a surgeprotection circuit, connected to said voltage input circuit, whichreceives an input voltage; a rectifier circuit, connected to said surgeprotection circuit, which receives an alternating current voltage fromsaid surge protection circuit and outputs a rectified direct currentvoltage; a transformer comprising first, second and third windings,which receives said rectified direct current voltage at said firstwinding so that current flows through said first winding, wherein saidsecond winding defines an unregulated output voltage of said powersupply, and wherein said third winding is substantially similar to saidsecond winding so that the voltage across said third winding is similarto the voltage across said second winding; a switching device, connectedto said first winding, for permitting and preventing the flow of currentthrough said first winding, wherein said switching device is operable inresponse to a control signal; and a controller, connected to saidswitching device, for generating said control signal based on thevoltage across said third winding, and wherein said control signaloperates to disable said switching member.
 2. The apparatus as recitedin claim 1, said controller comprising a current-mode regulator, whereina current reference signal is generated by said current-mode regulator.3. The apparatus as recited in claim 1, further comprising a linearregulator, wherein said output of said power supply is input to saidlinear regulator, wherein said linear regulator outputs a regulatedvoltage.
 4. The apparatus as recited in claim 3, wherein said regulatedvoltage is less than said output voltage, and wherein said regulatedvoltage is output to a precision voltage reference generator.
 5. Theapparatus as recited in claim 3, wherein said unregulated voltage isinput to said apparatus to determine the presence of a power failcondition.
 6. The apparatus as recited in claim 3, further comprising anon-volatile supply, wherein said regulated voltage is input to saidnon-volatile supply, wherein said apparatus is switched to saidnon-volatile supply when said regulated voltage is not present.